Ressa, who was named a Time Magazine "Person of the Year" in 2018 for her journalism, faces several criminal charges along with her website Rappler, in what press freedom advocates have branded an act of "persecution".

"Maria Ressa is a courageous journalist who is being persecuted for reporting the news and standing up to human rights abuses," Clooney said in a statement issued Monday by London's Doughty Street Chambers, where she works.

"We will pursue all available legal remedies to vindicate her rights and defend press freedom and the rule of law in the Philippines," added the British-Lebanese lawyer.

Clooney will work with a team of international lawyers based in the US and Britain as counsel to Ressa and coordinate with attorneys in Manila, the statement said.

Ressa was arrested twice this year and has accused Duterte of using prosecutions against her -- including ongoing cases of alleged tax evasion and libel -- to silence critics and intimidate the press.

Duterte has branded Rappler a "fake news outlet" and his government insists it is simply enforcing the law as cases pile up against the website, which frequently publishes reports critical of the president's deadly anti-drug crackdown.

"I am delighted that Amal Clooney and her team will be representing me at the international level to challenge the violations of my rights and those of the media organisation I represent," Ressa said in the statement.

Ressa said on Tuesday she faced at least 11 cases which required her to post bail eight times and pay a bond to travel overseas.

"Rappler should not have to face tactics of harassment, intimidation. We are trying to find the best way forward to make sure our rights are protected," Ressa told AFP.

Responding to the announcement, Duterte's spokesman said Clooney could not practise law in the Philippines but added he was "excited" to debate with the lawyer whom he described as "beautiful and sexy".

"Attorney Amal is misinformed. Maybe when we see each other, I can, to use the word of the president, educate her," Salvador Panelo told reporters.

Clooney, appointed special envoy for media freedom by the British government, also defended two Reuters journalists jailed for more than 16 months in Myanmar and freed in May.

Clooney's taking on Ressa's case will increase international attention on the journalist and her website, which have received a series of global awards from press freedom advocates.